Myeloid Derived Suppressor Cells (MDSCs) represent an intrinsic part of the myeloid-cell lineage that were initially described in cancer patients more than two decades ago. MDSC consist of a heterogeneous population of immature cells that expand during cancer, inflammation and infection. In particular, human MDSC are currently classified into granulocytic CD33+, CD15+, CD14neg, CD11b+, HLA-DRneg/low (G-MDSC) or monocytic CD33+, CD11b+, CD14+, CD15neg, HLA-DRneg/low (M-MDSC) sub-populations.
Monocytes are produced in the bone-marrow, and in healthy individuals they quickly differentiate into mature granulocytes, macrophages or dendritic cells (DC). However, under chronic inflammatory conditions such as in cancer, monocyte differentiation can be blocked which results in an expanded M-MDSC population found in peripheral blood. Currently the only approach to specifically isolate M-MDSCs is to islote CD14+ cells with low expression of HLA-DR and test if they can functionally suppress proliferation. However, the absence of positive markers specific for human M-MDSC leads to delays in the characterization and in-tissue study of this heterogeneous immunosuppressive cell population.